The document discusses absorption in the large intestine. It notes that the large intestine absorbs water and electrolytes like sodium, chloride, and potassium. Short-chain fatty acids, vitamins, and minerals are also absorbed through the large intestine due to digestion by gut bacteria. Absorption is regulated by hormones like aldosterone and the autonomic nervous system. The large intestine propels contents through haustral shuttling and mass movements to facilitate absorption and storage of waste as feces.

1. THE LARGE INTESTINE
PHYSIOLOGY: GASTROINTESTINAL

2. ABSORPTION IN THE
LARGE INTESTINE

3. ABSORPTION IN THE LARGE INTESTINE
OVERVIEW
‣ The large intestine, or colon, has several roles including water
absorption and immunity
‣ The chyme that enters the colon is already very concentrated as
most of the water has already been absorbed
‣ The large intestine is specialized to work in the environment this
produces
‣ Learning Goal
‣ To review the functions of the intestine and how they are
achieved

4. ABSORPTION IN THE LARGE INTESTINE
SPECIALIZED FUNCTIONS
‣ The large intestine is lined by mucosa with crypts of Lieberkühn containing glands and
mucus-producing goblet cells
‣ These protect the intestinal wall from the plethora of anaerobic bacteria in the colon and
from the pressure exerted on the walls by the concentrated chyme (soon to become
feces)
‣ The walls also contain gastrointestinal lymphoid tissue (GALT) that contributes to the
body’s immune defences
‣ The colon helps to absorb a small volume of water from the lumen (400ml/day)
‣ As the chyme is very concentrated by the time it reaches here, the colon must work
against a larger osmotic pressure gradient than in the rest of the GIT (it must move water
against the gradient for osmosis)
‣ The colon also helps to transport ions

5. https://teachmephysiology.com/gastrointestinal-system/large-intestine/

6. ABSORPTION IN THE LARGE INTESTINE
WATER AND ION ABSORPTION
‣ In the large intestine, there is a net absorption of sodium and chloride ions and
secretion of bicarbonate
‣ Sodium – this ion may be absorbed by various methods:
‣ Sodium-hydrogen antiporter on the luminal membrane
‣ Epithelial sodium channels
‣ Enhanced by absorption of short-chain fatty acids in the colon via
specialized symporters
‣ Chloride - the movement of sodium into the plasma produces an electrochemical
gradient to allow absorption of chloride
‣ Bicarbonate - chloride ions are exchanged for bicarbonate ions causing secretion

7. ABSORPTION IN THE LARGE INTESTINE
WATER AND ION ABSORPTION
‣ Water – the absorption of these electrolytes creates an osmotic gradient to allow further
absorption of water
‣ Potassium – absorption of water along the length of the bowel concentrates potassium in the
lumen
‣ This provides an electrochemical gradient for the movement of potassium into the plasma
‣ In the colon potassium may be absorbed or secreted depending on the remaining
concentration in the lumen and the electrochemical gradient created by the active
absorption of sodium
‣ Secretion usually occurs when the luminal concentration of potassium ions is below 25mM
‣ Vitamins and fats – short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K
(required for blood clotting) are produced by the digestion of chyme by the commensal
microbial flora of the colon

8. ABSORPTION IN THE LARGE INTESTINE
REGULATION OF ABSORPTION
‣ Absorption in the gastrointestinal tract is regulated by neuroendocrine mechanisms
‣ In the colon endocrine mechanisms used include:
‣ Aldosterone – increases the net absorption of water and electrolytes by stimulating the
basolateral sodium-potassium ATP-ase
‣ This increases the electrochemical gradient and driving force for sodium absorption
‣ It also increases transcription of epithelial sodium channels
‣ Glucocorticoids and somatostatin – act to increase water and electrolyte absorption by
increasing the action of the basolateral sodium-potassium ATP-ase
‣ The intestines are innervated by the enteric nervous system whose:
‣ Parasympathetic innervation promotes net secretion from the intestines
‣ Sympathetic innervation promotes net absorption from the intestines

9. ABSORPTION IN THE LARGE INTESTINE
REVIEW QUESTIONS
‣ How is Vit K produced and what is it necessary for?

10. ABSORPTION IN THE LARGE INTESTINE
REVIEW QUESTIONS
‣ How is Vit K produced and what is it necessary for?
‣ Vitamins and fats – short-chain fatty acids, crucial B
vitamins (such as B6 and B12) and vitamin K (required
for blood clotting) are produced by the digestion of
chyme by the commensal microbial flora of the colon

11. MOTILITY OF LARGE
INTESTINE

12. MOTILITY OF LARGE INTESTINE
OVERVIEW
‣ The large intestine is the final section of the
gastrointestinal system before the rectum
‣ In this section of the GI tract water is reabsorbed and any
remaining waste material is stored as feces to be removed
‣ Learning Goal
‣ To consider how waste material is moved through the
large intestine and clinical conditions that are relevant
to its function

13. MOTILITY OF LARGE INTESTINE
HAUSTRAL SHUTTLING
‣ The large intestine is naturally separated into segments known as haustra
‣ Along the course of the walls are groups of cells called pacemaker cells
‣ These send signals to the smooth muscle cells on the walls of the large intestine
causing them to contract at regular intervals
‣ The contraction causes the food to be churned in the intestine exposing the gut
contents to a larger surface area of epithelium maximizing absorption
‣ Each group of cells control a certain number of haustra
‣ The pacemaker cells closer to the ileum emit signals slightly faster than those
towards the end of the length of bowel
‣ This gradient allows a gentle progression of bowel contents towards the rectum

14. MOTILITY OF LARGE INTESTINE
MASS MOVEMENT
‣ Whilst haustral shuttling occurs continuously mass movement only
occurs once or twice per day
‣ This involves a sudden, uniform peristaltic contraction of smooth
muscle of the gut which originates at the transverse colon and
rapidly moves formed feces into the rectum, which is normally
empty
‣ The result of this is feeling the urge to defecate
‣ The contraction may be stimulated by eating
‣ When this occurs it is called the gastro-colic reflex

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17. https://teachmephysiology.com/gastrointestinal-system/large-intestine/large-intestinal-motility/
FBC - full blood count / U&E - urea and electrolytes / LFTs - liver function tests / CRP - C-reactive
protein / ESR - erythrocyte sedimentation rate

18. LARGE INTESTINAL MOTILITY
CLINICAL SIDE NOTE: IBS VS IBD
‣ But despite having similar acronyms and symptoms, these
two conditions are very different
‣ IBS is a disorder of the gastrointestinal (GI) tract
‣ IBD is inflammation or destruction of the bowel wall, which
can lead to sores and narrowing of the intestines
‣ It's possible to have both IBD and IBS

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25. LARGE INTESTINAL MOTILITY
REVIEW QUESTIONS
‣ What does the blood test fecal calprotectin help
differentiate between?
‣ Ulcerative colitis and Crohn's
‣ Ulcerative colitis and IBS
‣ Crohn's and IBS
‣ IBD and IBS

26. LARGE INTESTINAL MOTILITY
REVIEW QUESTIONS
‣ What does the blood test fecal calprotectin help differentiate
between?
‣ Ulcerative colitis and Crohn's
‣ Ulcerative colitis and IBS
‣ Crohn's and IBS
‣ IBD and IBS
‣ Faecal calprotectin is a good indicator between IBD and IBS, but it
cannot differentiate between different types of inflammation seen in
Crohn's and UC

27. DEFECATION

28. DEFECATION
OVERVIEW
‣ The large intestine is the final section of the
gastrointestinal system before the rectum
‣ In this section of the GI tract water is reabsorbed and any
remaining waste material is stored as feces to be removed
‣ Learning Goal
‣ To consider the physiological process of defecation
and clinical conditions that may occur during
dysfunction

29. DEFECATION
RECTUM
‣ The rectum is responsible for temporary storage of feces before
defecation
‣ As it becomes filled the rectal walls expand and stretch
receptors stimulate the desire to defecate
‣ The urge to defecate arises from contraction of rectal muscles,
relaxation of the internal anal sphincter and an initial
contraction of the external anal sphincter
‣ If the urge is not acted upon further water is absorbed and the
feces is stored until the next mass movement occurs

30. https://teachmephysiology.com/gastrointestinal-system/large-intestine/defecation/

31. DEFECATION
DEFECATION
‣ There are two main anal sphincters: internal and external
‣ The internal anal sphincter is controlled by
parasympathetic fibres which relax involuntarily
‣ The external anal sphincter is skeletal muscle that is
controlled by somatic nerve supply
‣ Inferior anal branch of the Pudendal nerve (S2,3,4)
‣ Allows conscious control of defecation

32. DEFECATION
DEFECATION
‣ When the rectum is distended the rectosphincteric reflex is initiated
and relaxes the internal sphincter
‣ If defecation is not desired, voluntary contraction of the external
sphincter can delay it
‣ If defecation is appropriate, then a series of reflexes takes place that
leads to:
‣ Relaxation of the external sphincter
‣ Contraction of abdominal wall muscles
‣ Relaxation of pelvic wall muscles

33. DEFECATION
DEFECATION
‣ Peristaltic waves then facilitate the movement of feces
through the anal canal
‣ Defecation can also be assisted by taking a deep breath
and attempting to expel the air against a closed glottis,
this is known as the Valsalva maneuver
‣ However, if a delay in defection is needed then voluntary
contraction of the external sphincter is usually sufficient to
override the reflexes that anal distension initiates

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36. https://teachmephysiology.com/gastrointestinal-system/large-intestine/defecation/

37. DEFECATION
REVIEW QUESTIONS
‣ Conscious Control of Defecation is controlled by which
nerve?
‣ Pudendal nerve (S2, S3, S4)
‣ Ilioinguinal
‣ Genitofemoral nerve
‣ Sacral nerve

38. DEFECATION
REVIEW QUESTIONS
‣ Conscious Control of Defecation is controlled by which
nerve?
‣ Pudendal nerve (S2, S3, S4)
‣ Ilioinguinal
‣ Genitofemoral nerve
‣ Sacral nerve

39. DEFECATION
REVIEW QUESTIONS
‣ What can cause fecal incontinence?
‣ Childbirth
‣ Caffeine
‣ Alcohol abuse
‣ Opioids

40. DEFECATION
REVIEW QUESTIONS
‣ What can cause fecal incontinence?
‣ Childbirth
‣ Caffeine
‣ Alcohol abuse
‣ Opioids

41. References
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defecation/
‣ https://teachmephysiology.com/gastrointestinal-system/large-intestine/large-
intestinal-motility/
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